The goal of putting more and more devices into integrated circuits has been important in semiconductor manufacturing from the very invention of the integrated circuit. Higher density traditional circuits allow for the manufacture of ever more powerful devices while greatly reducing costs on a per transistor basis. The traditional configuration of circuit elements is to form them laterally on the surface of a semiconductor substrate. This provides ease of manufacturing and reduced complexity. However, currently, semiconductor design engineers are struggling with many limitations of lateral devices.
One great challenge is the limits of lithography. The layers of integrated circuits are typically fabricated by patterning various components using photolithography. In photolithography, a layer of photo-sensitive material called photoresist is coated onto the device. Then, the photoresist is exposed to a light pattern corresponding to the desired patterns in a particular layer. However, components in integrated circuits have become so small that their size is on the order of the wavelength of light used to expose the photoresist. While various techniques have been employed to push this limitation beyond all reasonable expectation, at some point, this physical limitation will become insurmountable. Engineers continue to devise structures to overcome this and other physical limitations.